This invention relates to electro-fusion fittings and their manufacture. Electro-fusion fittings are used for making couplings in thermoplastics, most commonly polyethylene pipe. The invention is particularly concerned with fittings intended for connecting two pipe lengths end-to-end, such fittings being frequently referred to as welding muffs or sleeves. Conventional welding sleeves consist of a hollow cylindrical body of thermoplastic material with a coil of electrical resistance heating wire at its inner surface. In use, the pipe ends are pushed into the opposite ends of the sleeve and an electric current is supplied to the heating coil so that the material of the pipes and fitting body is melted and fuses together to form a secure, leakproof coupling between the pipe ends.
Different methods can be employed for manufacturing electro-fusion sleeves. Most convenient in the case of sleeves of relatively small diameter, and hence having bodies of relatively small mass, is a technique wherein the body is produced by a normal injection moulding process. The heating wire, coated in the thermoplastic material, may be wound onto an injection mould core and then have the body injection moulded in situ over the wire coil.
This technique is not ideal for manufacturing large diameter fittings, however, because tooling costs are very high and the production rate is very low due to a long cycle time (in the order of 20 minutes) which arises through the time it takes to fill the mould cavity with molten plastic and the subsequent curing time necessary before the mould can be opened. As a solution to the problem of producing economically large diameter fittings, it has been proposed in GB-A-2036518 to form the body by cutting a length off from a pipe of thermoplastic material. The heating coil is applied to the interior of the pipe length by a process which involves: expanding the pipe length and introducing into it a mandrel onto which the wire, either bare or coated with thermoplastic material, has been wound; the pipe length is shrunk onto the mandrel and winding in a furnace; electric current is supplied to the heating wire to cause the wire turns to expand while the mandrel is expanded or shrinking of the pipe length is continued, and the thermoplastic material is caused to flow between the turns; and, after cooling, the wire ends are connected to contact pins previously mounted on the pipe length. Thus, the procedure is complicated and there still exists a need for a reliable and economic manufacturing method.